To permit clean stereo reception in a radio, for example a car radio, the field strength of the received signal must be sufficiently great. If the field strength is too low there is acoustically unpleasant interference. Therefore many car radios have a switchover key for switching to mono operation when the reception worsens due to deficient field strength. In mono operation there is no, or at least less, interference. The user of the radio performs the switchover by hand when he feels the reception is too poor. Later he can switch back to see whether good stereo reception is possible again.
One can think of performing the switchover automatically in the device itself rather than manually. For this purpose one can compare the field strength signal acquired in the RF section of the radio e.g. with a threshold in order to switch to stereo operation or mono operation in accordance with the amplitude of the field strength signal.
However, one can also perform this switchover process continuously, likewise in accordance with the amplitude of the field strength signal. In modem motor vehicle radios (and similar devices) the major part of the entire signal processing takes place in a combination of stereo decoder and audio processor. The stereo decoder formed as an integrated circuit is supplied by the RF section the output signal of the IF demodulator, as well as the above-mentioned field strength signal. Elimination of the pilot signal, channel separation, low-pass filtering for damping and the like then take place within the stereo decoder. The audio signal processor formed of stereo decoder and audio processor is furthermore supplied signals from a cassette part or compact disk player possibly present. Also, adjustment of volume, adjustment of treble and bass and the like take place in the audio signal processor.
If one provides in such an audio signal processor a simultaneously integrated circuit which performs continuous switching between stereo operation and mono operation in accordance with a field strength signal, by comparing the field strength signal with a reference triangular signal for forming a PWM signal and having this PWM signal drive high and hold low a signal path between input and output of the demodulator in accordance with its pulse-width repetition rate (ratio of pulse length to interpulse period), an adjustment must be made during assembly of such an audio signal processor in a radio (or, more generally, an audio device) in order to fix the two limits for the variable factor.
The factor varies continuously between the two limits. Above the upper limit there is full stereo operation, and below the lower limit there is solely mono operation.
These limits are set during alignment in the course of the manufacture of the audio device when the audio signal processor formed as an integrated circuit is built in. This alignment is necessary because the amplitude of the field strength signal supplied to the processor during operation is not standardized and can be subject to fluctuations from manufacturer to manufacturer. Alignment ensures that the continuous transition between mono operation and stereo operation actually takes place in optimal fashion, i.e. there is as little interference as possible and at the same time stereo operation takes place as soon as the existing field strength permits.
Hitherto it was usual to wire the chip of the audio processor externally with resistors during the above explained alignment, the manufacturer of the audio device then performing the alignment by means of suitably selected resistance values.
However, wiring an IC chip with external components is not only time-consuming and costly but corresponding connections must also be present on the chip or chip package.